This invention pertains to the art of electrical power generators and more particularly to high frequency and high power solid state inverter systems for supplying alternating current at frequencies in the range of 50 kHz.
The invention is particularly applicable for use as a power supply for an induction heating system for energizing a heating coil and will be described with particular reference thereto. However, it will be appreciated that the invention may have broader applications in various other industrial processes such as those that involve a varying impedance load, and may be advantageously employed in such other environments and applications.
High frequency power inverters for supplying energy to induction heating coils are well known. U.S. Pat. No. 3,821,632 shows one such system and is particularly referenced herein for its showing of a common attribute among such inverters, that is, a precharge circuit. FIG. 1 of the subject application shows a simplified schematic representation of the system shown in the '632 patent. Such a circuit is commonly referred to as a charge/discharge circuit ("C/D circuit") for reasons that will be evident from the following brief description of its operation. The load is generally designated with the numeral "10" and typically comprises a tank circuit that receives its power from transformer T1, although in some circumstances a transformer may not be needed. The load is represented by an LRC circuit. As noted above, the inverter supplies an alternating current to the load from a direct current power supply. In the first half cycle the supply voltage 14 charges capacitor Cl through inductor L1 when SCR1 is fired. (The circuit is described with reference to SCRs as switches, although many types of thyristors or other switch devices could be successfully employed.) This is the "charge" aspect of the inverter. Next SCR1 is turned OFF and SCR2 is turned ON to discharge C1 through L3 and the load. Similarly, the second half cycle is initiated by firing SCR3 which charges capacitor C2 through inductor L2. Then SCR4 discharges capacitor C2 through inductor L4 and the load. During subsequent steady state operation, if there is residual voltage on capacitor C1 in excess of the voltage level of supply 16, then diode D1 exerts a clamping action to maintain a controlled voltage on capacitor Cl limited to the supply voltage. Similar clamping action is maintained by diode D2 on capacitor C2. All thyristor firing pulses are generated from an essentially free running oscillator with its repetition rate being limited by any number of circuit limit conditions. The inductances represented by L5 and L6 are unavoidable leakage inductances which are typically minimized as much as possible to avoid inhibiting the clamping action of diodes D1 and D2. The inductances L3 and L4 are chosen to form a discharge current pulse with an appropriate width for the load resonant frequency, while the inductances L1 and L2 are chosen for an appropriately fast charge of the capacitors C1 and C2 within the limit of the SCR1 and SCR3 DI/DT capabilities.
In summary, the circuit clearly includes a precharge circuit which does not include the load and only delivers energy from the supply to the charge capacitors C1 and C2. The discharge portion of the circuit discharges C1 and C2 into the load where the energy is partially dissipated; that is, in order to deliver the energy of the DC supply to the load a charge/discharge action must take place. This results in undesirable losses occurring both in the charge and discharge circuit for every unit of energy delivered to the load.
The perceived advantages of such a circuit are that the charge and discharge circuits are isolated for safety of operation and the clamping circuit provides a controlled limit for the ring-up voltage on the discharge capacitor C1.
Several notable problems exist with such C/D inverters. First, it is readily apparent that the circuit is complicated in its timing operation and because of its use of so many components. Second, it is inefficient in that for each and every half cycle of power delivered to the load, associate losses are incurred in the necessary precharge of the inverter. Third, a load matching problem exists. FIG. 3 of the application represents the load matching range of a prior art C/D inverter. The significance of this FIGURE is that for an induction heating load, as the load workpiece varies in temperature, its conductance will vary on the abscissa of the graph. The narrower the full power delivery curve is at the peak, the less likely it is for the inverter to deliver full power to the load as the load condition changes.
The present invention contemplates a new and improved inverter which overcomes all of the above referred to problems and others to provide a new high frequency power inverter which is simple in design, requires a reduced number of components, has a wide range load matching capability readily adaptable to a plurality of loads and will still provide full power, is highly stable and can dependably assure a reverse voltage to be applied to the thyristors cyclically.